A known drying device for a car wash can have a light gate controlling the spacing between the vehicle body and the nozzles discharging the drying air. This gate can have a varying response which can be altered by sprayed water or a dirt condition increasing the light attenuation between the light source and light detector. The light gate is also influenced by reflected or ambient light or by temperature or other changes of component characteristics.
When an incomplete light beam interruption occurs on entrance of the vehicle into the light beam, particularly on entrance of the curved vehicle front or rear panel into the beam, frequently only an insignificant beam attenuation is noted so that the gating signal for "beam breaking" and/or "beam release" fails to occur or is too late so that the portion of the drying device controlled by the gating signal is not kept a sufficiently safe distance from the contour of the vehicle. In such units the response sensitivity of the gating must be adjusted manually by changing the response threshold of the threshold circuit or by changing the amplification of the limiting signal or by changing the light intensity of the light source.
In practice during operation, manual adjustment of the response sensitivity has a big disadvantage, namely that frequently an incorrect sensitivity is used since the adjustment must be made by a novice or technically unskilled person. Severe damage can also occur when the gating does not respond properly to the entrance and exit of the components of the vehicle into the light beam.
A vehicle wash unit is known according to German Open Patent Application No. 32 08 527 in which a vehicle moves past several work stations, the contour of the vehicle already being determined and stored before the vehicle reaches the first work station by a sensing device comprising a plurality of light gates positioned one above another. The supplied contour data is then used by the individual work stations for positioning of their working means as the vehicle passes through. This is a complex arrangement not amenable to use with frame-type car washers which are moved along the vehicle.